Vibrating conduit sensors, such as Coriolis mass flow meters, typically operate by detecting motion of a vibrating conduit that contains a material. Properties associated with the material in the conduit, such as mass flow, density and the like, in the conduit may be determined by processing signals from motion transducers associated with the conduit, as the vibration modes of the vibrating material-filled system generally are affected by the combined mass, stiffness, and damping characteristics of the conduit and the material contained therein.
A typical Coriolis mass flow meter includes one or more conduits that are connected inline in a pipeline or other transport system to convey material, e.g., fluids, slurries and the like, in the system. Each conduit may be viewed as having a set of natural vibration modes including, for example, simple bending, torsional, radial, and coupled modes. In a typical Coriolis mass flow measurement application, a conduit is excited in one or more vibration modes as a material flows through the conduit, and the motion of the conduit is measured at points spaced along the conduit. Excitation is typically provided by an actuator, e.g., an electromechanical device, such as a voice coil-type driver, that perturbs the conduit in a periodic fashion. Mass flow rate may be determined by measuring the time delay or phase differences between motion at the transducer locations.
The magnitude of the time delay is very small; often measured in nanoseconds. Therefore, it is necessary to have the transducer output be very accurate. Transducer accuracy may be compromised by nonlinearities and asymmetries in the meter structure or from motion arising from extraneous forces. For example, a Coriolis mass flow meter having unbalanced components can vibrate its case, flanges, and the pipeline at the drive frequency of the meter. This vibration perturbs the time delay signal in an amount that depends on the rigidity of the mount. Since the rigidity of the mount is generally unknown and can change over time and temperature, the effects of the unbalanced components cannot be compensated and may significantly affect meter performance. The effects of these unbalanced vibrations and mounting variations are reduced by using flow meter designs that are balanced and by using signal processing techniques to compensate for unwanted component motion.
Typical dual tube Coriolis flow meter designs split the flow of material into two streams using manifolds and send the two streams of material into the flow tubes. The two tubes are typically symmetrical in shape and mounted parallel to one-another. The two tubes typically vibrate at the same frequency but in opposite phase. Because the tubes are symmetrical and vibrated opposite each other, the vibrations typically cancel out where the two tubes are joined. This creates a balanced flow meter (i.e., little or no vibration of the meter at the manifolds). A change in density in the material flowing through the two tubes changes the mass of both tubes equally and therefore, the two tubes remain balanced across a wide range of material densities.
There are certain applications where dual tube meters are not wanted due to pressure drop or plugging issues, in these cases a single tube meter is desirable. The problem with single tube Coriolis flow meters is that they can become imbalanced with changing fluid densities. As the fluid density changes the center of mass of the flow meter also changes. This imbalance can have adverse effects on the meter's performance and reliability.
Therefore, there is a need in the art for a single tube Coriolis flow meter that is capable of staying balanced over a wide range of material densities. The present invention overcomes this and other problems and an advance in the art is achieved. It should be appreciated however, that while the present invention overcomes difficulties that are particularly prevalent with single tube designs, the invention is equally applicable to dual tube meters.